I have a spreadsheet, these are my headings:
Shop Date
MSC
Client Name
Address/Town
Approved (x) goes in this column when approved
Date paid
Job info/brief description/notes
Payment amount
Reimbursement amount
Purchase Amount
I used to track mileage on there but now I just keep a mileage book in my car, it is easier to figure out for me.
I keep a tab for each month and a summary sheet that pulls from the other tabs
I also have a tax sheet where I track expenses, like paper or ink or any type of supply I purchase such as a notebook or calendar book, pens, sharpies, safety vest, stuff like that. Basically anything purchased for a job.
Hope that helps